This invention relates to zeolites that are useful as adsorbents or catalyst supports. In particular, it involves production of a hydrophobic zeolite.
Most zeolites are hydrophilic (water attracting) and thus have higher preference for sorption of water than for organic materials. However, the highly siliceous zeolites tend to be hydrophobic (organic-attracting). Hydrophobic zeolites are useful in selected applications such as removal of volatile organic compounds from water-containing environments.
Hydrophobic zeolites tend to have a relatively small number of catalytically active acid sites. These low acidity zeolites are sometimes useful in catalytic processes where cracking reactions must be minimal.
In order to measure the hydrophobicity of a zeolite, we have developed a Hydrophobicity Index screening test. A Hydrophobicity Index (H) is calculated from the ratio of mass sorption of organic compound to mass sorption of water at specific partial pressures for the two adsorbates; thus Hc=Sc/Sw for cyclohexane over water and Hn=Sn/Sw for n-hexane over water. Highly hydrophilic zeolites will have H values of less than 1.0. Highly hydrophobic zeolites will have H values of substantially greater than 1.0. Selection of the adsorbent depends upon the pore opening of the zeolite structure of interest. It is well known that zeolites with 10-membered or less metal atoms ring openings will not adsorb substantial amounts of cyclohexane. For these zeolites, e.g. ZSM-5, ZSM-11, etc., n-hexane is much more efficacious choice for the organic adsorbent. Moreover, the partial pressure at which the adsorbtion is measured can have an effect on the absolute amount of adsorption of any component and also the hydrophobicity index value. For the purpose of defining the conditions at which the index is measured (the adsorbate and the partial pressures) we have adopted the following convention: Hc07/05 refers to an index where cyclohexane adsorption at 7 torr is referenced to water adsorption at 5 torr. Similarly, Hn07/05 refers to an index where n-hexane adsorption at 7 torr is referenced to water adsorption at 5 torr.
A hydrophobic zeolite can be prepared by calcining a precursor zeolite with silica to alumina molar ratio at least 20, under high temperature and the presence of steam and under turbulent conditions with respect to flow pattern of the zeolite. In particular, a novel hydrophobic zeolite Y is provided by this method having a Hydrophobicity Index (Hc07/05) of greater than 20.
We have found that by calcining zeolites under a turbulent condition, high temperature and in the presence of steam, a hydrophobic zeolite can be prepared. Turbulent condition arises from intimate admixture of the solid and the gas phase such that the characteristic flow pattern of the solid can be considered turbulent. These zeolites are more hydrophobic than zeolites that can be prepared by steam calcining a zeolite under non-turbolent conditions. Examples of hydrophobic zeolites that can be prepared by this method include, for example, zeolite Y, and zeolite beta. These zeolites are considered to have interconnecting pores of at least two-dimensions, preferably interconnecting two or three-dimensions, more preferably three-dimensions. The precursor (starting material) zeolites useful in preparing the hydrophobic zeolites have a silica to alumina molar ratio of at least 20, preferably from about 25, to about 150. The calcination temperature is in the range of from about 650xc2x0 C., preferably from about 700xc2x0 C., to 1000xc2x0 C., preferably to 850xc2x0 C. in the presence of steam. The steam is preferably present in an amount of at least 10% by volume.
In particular, we have found that by preparing the zeolite by calcining a zeolite having silica to alumina greater than 20, particularly stabilized zeolite Y under a turbulent condition, high temperature and in the presence of steam, a hydrophobic zeolite, particularly a stabilized zeolite Y having a Hydrophobicity Index (Hc07/05) of greater than 20, preferably at least 25, can be prepared.
The very hydrophobic zeolite products of our invention are prepared from zeolites having the structure of zeolite Y that is stabilized. These very hydrophobic zeolites have Hydrophobicity Index (Hc07/05) of greater than 25, preferably greater than 30: The ultrahydrophobic materials have a Hydrophobicity Index (Hc07/05) of greater than 30, preferably equal to or greater than about 35.
It has been surprisingly found that a very hydrophobic zeolite Y material can be prepared from a precursor material with a moderate silica to alumina molar ratio (bulk silica to alumina ratio) in the range of from 25, preferably from about 40, to about 150, preferably to about 120.
It has also been surprisingly found that an ultrahydrophobic zeolite Y material can be prepared from a precursor having silica to alumina molar ratio of greater than about 60, preferably greater than about 75, preferably greater than about 85.
The hydrophobic zeolite Y material of the invention can be produced by calcining a stabilized Y zeolite having a unit cell size within the range of less than 24.40 preferably less than 24.35, more preferably less than 24.30, most preferably less than 24.27, to preferably greater than 24.15, under turbulent conditions at a temperature within the range of from about 650xc2x0 C., preferably from about 700xc2x0 C., to 1000xc2x0 C., preferably to 850xc2x0 C. in the presence of steam. The steam is preferably present in an amount of at least 10% by volume.
Turbulent condition as herein referred to is a condition in which there is sufficient mix between solid phase and gas phase in which the gas flows through the dispersed solid phase without a discernable interface. The condition is not turbulent if the gas phase flows over a stationary solid such that there is a discernable interface between the solid and the gas.
While not wishing to be bound by theory, we believe that superior contacting of the solid involved with the reactive gas atmosphere directly leads to the high hydrophobicity characteristic of the present invention. We believe that this condition is met when a substantial portion of the solid particles are continuously and completely surrounded by the reactive gas mixture. This condition can be described as a flow rate such that a significant fraction of the solid articles have reached the point where they have at least just been suspended and set in motion by the action of the gas. Such a velocity has often been described as the minimum fluidization velocity. This often occurs at Reynolds numbers (NRe) less than about 10 (DpGmf/xcexc). This phenomenom has been described by the following relationship (Leva, xe2x80x9cFluidization,xe2x80x9d p. 63, McGraw-Hill, New York 1959):       G    mf    =            0.0005      ⁢              D        p        2            ⁢              g        c            ⁢                        ρ          f                ⁡                  (                                    ρ              s                        -                          ρ              f                                )                    ⁢              φ        s        2            ⁢              ϵ        mf        3                    μ      ⁡              (                  1          -                      ϵ            mf                          )            
where
Gmf=fluid superficial mass velocity for minimum fluidization, lb./(sec.)(sq.ft.)
Dp=particle diameter, ft.
gc=dimensional constant, 32.17 (lb.)(ft.)/(lb.force)(sec.2)
xcfx81f=fluid density, lb./cu.ft.
xcfx81s=solids density, lb./cu.ft.
"PHgr"s=particle shape factor, dimensionless
xcex5mf=voidage at minimum fluidization, dimensionless
xcexc=fluid viscosity, lb./(ft.)(sec.)
Alternately, this has been described by a similar equation (Perry, xe2x80x9cChemical Engineers"" Handbook,xe2x80x9d 4th Edition, p. 4-25, McGraw-Hill, New York):       G    mf    =            5.23      xc3x97              10        5            ⁢              D        p        2            ⁢                        ρ          f          1.1                ⁡                  (                                    ρ              s                        -                          ρ              f                                )                      μ  
where
Gmf=fluid superficial mass velocity for minimum fluidization, lb./(hr.)(sq.ft.)
Dp=particle diameter, ft.
xcfx81f=fluid density, lb./cu.ft.
xcfx81s=solids density, lb./cu.ft.
xcexc=fluid viscosity, lb./(ft.)(sec.)
For the invention process, it is preferable to calcine under a minimum fluidization velocity through at least substantial portion of zeolite particles in contact with gas phase where flow rate has a Reynolds number of at least 5, preferably at least 10. Substantial portion of zeolite particles are in contact with the gas phase when at least 50%, preferably 85%, more preferably 95%, most preferably 100% of the zeolite particles are in contact with the gas phase.
To produce a turbulent condition, for example, a fluidized bed calciner or ebulating bed calciner, such as those available from such companies as Procedyne (New Brunswick, N.J.) and A. J. Sackett and Sons (Baltimore, Md.), and others can be used. This is not meant to be an exhaustive list of equipment but only to provide description of the types of equipment that are suitable for the process described.
The equipment should be operated with sufficient flow of gas phase to produce turbulence in the solid and at a temperature and steam partial pressure effective to produce a hydrophobic zeolite of the invention.
The starting stabilized zeolite Y can be prepared from zeolite NaY. Zeolite NaY can be produced by any conventional manner from water, a source of alumina, a source of silica, and sodium hydroxide. The resulting NaY zeolite has silica to alumina molar in the range of 4.0 to 6.0. Stabilization of this material is accomplished by combination ion exchange and steam calcination with at least one step of each. One way to prepare such zeolite is described in U.S. Pat. No. 5,059,567 which disclosure is hereby incorporated by reference and another in U.S Pat. No. 4,477,336, which disclosure is also incorporated by reference. In one method to prepare the starting material, the NaY can be ion-exchanged with ammonium solution, such as ammonium sulfate one or more times, washed and dried. The ammonium ion-exchanged zeolite can be calcined at a temperature in the range of 550xc2x0 C. to 800xc2x0 C. in the presence of steam. This zeolite is then further ion exchanged with an ammonium solution and then recalcined in a similar temperature range. Following this calcination, the resultant zeolite is dealuminated by contact with mineral acid under conditions such that the desired silica to alumina molar ratio is achieved.
The novel hydrophobic zeolite Y of the invention has a unit cell size in the range of from 24.15, preferably from 24.20, to 24.35, preferably to 24.28 angstrom. The surface area of these novel hydrophobic zeolite Y materials is preferably at least 500, more preferably at least 600 square meters per gram. The silica to alumina (chemical) molar ratio is substantially unchanged from the stabilized precursor zeolite. Organic adsorption is at least 10% by weight at a pressure of 7 torr.
Without wishing to be bound by any particular theory we speculate that the exceptionally high degree of hydrophobicity obtained in zeolite materials by the process of this invention may be due to the changed nature of the interaction between the zeolite solids and the reactive gas atmosphere. In the turbulent fluid bed the degree of intimacy of contact between all solid particles and the hydrothermally reactive gas phase is much greater than can be effected in a static bed or in the mildly roiled bed of powder found in a rotary kiln. The typical rotary kiln is operated as a continuous process with a constant feed of powder entering one end of a heated tube and a constant flow of processed material discharged from the other. The steam or air/steam mixture used in a counter-current flow to treat the zeolite powder, according to the teachings of prior art, largely passes over the bed of solids making its most effective contact only with the solids exposed at the bed surface by the slow turnover with rotation of the calciner tube. In such equipment excessive turbulence must normally be prevented so as to avoid entrainment of solids in the gas phase and loss of material from the kiln.
It is known that the combination of water and high temperature promotes the hydrolysis of framework Al out of the zeolite structure thus eliminating framework charge centers according to the following reaction:
[AlO4xe2x88x92], H++3H2O=[(OH)4]+Al(OH)3xe2x80x83xe2x80x83(1)
where [AlO4xe2x88x92] indicates the anionic charge center in the tetrahedral framework lattice and [(OH)4] indicates the xe2x80x9chydroxy-nestxe2x80x9d framework vacancy created by hydrolysis to generate non-framework Al(OH)3 and related species. The xe2x80x94OH groups in the vacancy are attached to Si atoms in the lattice. This treatment removes sites at which polar water molecules can be held by ion dipole interaction.
Water can also be bound to the solid by interaction with residual hydroxyl groups (xe2x80x94OH) by H-bonding. Thermal treatment alone eliminates most hydroxyl groups in zeolite materials at temperatures between 500-650xc2x0 C. as indicated by TGA analysis. The hydroxyl elimination reaction can be written in simplest form as:
xe2x89xa1Sixe2x80x94OH+HOxe2x80x94Sixe2x89xa1=xe2x89xa1Sixe2x80x94Oxe2x80x94Sixe2x89xa1+H2Oxe2x80x83xe2x80x83(2)
However, we have discovered that this reaction is not sufficient to substantially complete the elimination of hydrophyllic centers for zeolites. We have discovered surprisingly that contacting the zeolite with steam under turbulent conditions, at temperature of above 650xc2x0 C. with steam produces zeolites that are highly hydrophobic. We now speculate that the forced elimination of hydroxyl groups at high temperature can generate significant strain in the siloxane bonds thus formed.
Framework vacancies are especially susceptible to the formation of strain centers. Such strained bonds possess varying degrees of partial polarization and this residual polarity provides sites for sorption of water and reversal of reaction (2) when the zeolite material is exposed to aqueous vapor again under milder thermal conditions. To minimize such polarized strain sites within the zeolite structure it is necessary to promote the release of strain to a substantial degree by a time dependent annealing process. The annealing mechanism may involve a continuous and reversible breaking and forming of bonds allowing the whole crystal structure to undergo a progressive relaxation towards minimized residual strain. This mechanism is most effectively catalyzed by the well known xe2x80x9cmineralizing actionxe2x80x9d of water vapor.
Hydrothermal treatment of zeolite materials in a turbulent fluidized bed without tangibly identifiable phase boundaries appears to drive the hydrophobization process in zeolite materials to a degree that has not been recognized previously and which is not attainable by treatment of non-fluidized material for comparable times under equivalent hydrothermal conditions. Since we believe that the optimum annealing process involves a uniform minimization of residual strain energy throughout the structure of each crystal there is reason to suppose that the mechanism will be most effective for treatments that expose zeolite particles and particularly crystals to uniform conditions of heat transfer and contact with water vapor that is independent of any direction in space due to equipment or powder bed configuration. While it would not be surprising to find differences in the response of different crystal structures to this isotropic environmental condition we believe the treatment given to zeolites by the process of this invention is expected in every case to move the material in the direction of enhanced hydrophobicity. These novel zeolites can be useful as adsorbent for organics.